Solar energy collectors to produce the high fluid temperatures required for power generation are based on well-known principles of reflectivity, and, in certain instances, the use of heliostat tracking similar to that described in U.S. Pat. No. 2,712,772.
An experimental "Power Tower" nearing completion in Barstow, California uses more than 1700 heliostats (position controlled mirrors), each having a toal area of 450 sq. ft., and each being independently pivotable biaxially to reflect solar rays to a distant receiver (reference Machine Design, July 12, 1979, pages 20-25). The solar energy power generating installation described will deliver 10 megawatts of power at an installed cost of between $10,000 to $15,000 per KW as compared with a range of about $750 to $1500 per KW for conventional coal fired or nuclear installations (1980 costs).
Design aspects of the "Solar Power Tower" are described in the Mar. 19, 1979 issue of Design News Aerospace Technology--"To gather 300 Suns".
These articles clearly indicate that heliostat design is neither simple, nor a fully matured art, and discuss problems associated with proper focusing of heliostats within accuracies of 6-10 milliradians to achieve fluid heating to about 890.degree. F. Not the least of the design problems is the fact that the mirrors, their pedestals, and their drives must withstand high winds and earthquakes. It is noteworthy that using a large number of heliostats to concentrate solar rays onto a centrally located absorber tower is analagous to a stationary disc concentrator having a diameter equal to the peripheral dimension of the heliostat array, but with a requirement for space between each heliostat to keep it from being shaded and to allow enough space for rotational movement. Moreover, the heliostat array-central power tower represents inefficient land area usage. It is also noted that when viewed from outer space, the heliostat array is not symmetrical about the central absorber-tower, this arrangement being dictated by the fact that, as an equivalent to a large disc collector, it remains horizontal and small portions of the equivalent "disc reflector surface" are individually oriented to compensate for changes in solar angle.
As opposed to this, better utilization of the reflector area (and land use) can be achieved when the entire disc assembly and receiver can be tilted as an entity, even though there are practical limitations on the diameter of the disc and its pivoting or rotational movement about axes perpendicular to each other. "Combined" designs whereby the disc reflector and absorber are moved as an assembly are similar to that described in Machine Design of Mar. 6, 1980. These systems utilize reflector disc (paraboloids of revolution), which concentrate solar rays on an absorber mounted at the focal point of the reflector, and it is noted that because of better total reflector surface utilization (as opposed to the spaced-apart heliostats in the equivalent disc reflector of the "Power Tower"), higher concentrating ratios are possible and result in heat transfer fluid temperatures of 1500.degree. F. when focusing solar rays from a disc reflector of 33 ft. in diameter. Disc reflectors are well adapted for smaller power generation units that can be erected in many locations--often on otherwise unuseable plots of land,--and, power outputs can be integrated or readily connected to nearby and existing power grids.
The basic problem of high reflector cost with heliostats or disc-type reflectors resides in the requirement for special metal fabricating techniques to insure paraboloidic reflecting accuracy. Furthermore, due to the high accuracy and special techniques required, these reflectors must be fabricated at special facilities and then transported to the installation site as assembled or partially disassembled units due to carrier size, weight, and/or right-of-way overhead restrictions.
In this disclosure, I describe a disc reflector (paraboloid of revolution), essential components of which may be made on standard high-speed, corrugating machinery that is modified to shape parabolic supporting members according to methods similar to those described in my copending application, Ser. No. 12032.
In order to use parabolic shapes for concentrating disc reflectors, certain fundamental changes must be made to the shape and length of individual segments in order to allow for field assembly into a disc reflector of high paraboloidal accuracy.
In this regard, the previous disclosure of my U.S. Pat. No. 4,190,037 relates to similarly-shaped underlying supports. However, changes in fabricating methods and to the shape as well as arrangement of underlying supports are described as they apply to disc reflectors generically--for example, U.S. Pat. Nos. 2,460,482; 3,105,486; 3,162,189; 3,643,648; and 3,713,727.
Reflectors are currently being tested for solar ray reflection-absorption and useful heat transfer-conversion into electric power. For the purpose of distinguishing over prior art, high-intensity solar collectors, certain background information and data is discussed relative to a 33 ft. diameter reflecting disc, which, according to the above-mentioned article, can collect and convert enough solar rays for a 20 KW generating unit.
While a disc reflector having a 33 ft. diameter is described, it is understood that the principles of paraboloidal reflectivity and the methods of fabrication, transport, and assembly can apply to similar reflectors to the limits of practical size. For example, FIGS. 17 and 18 refer to disc reflectors that can be substantially larger than 33 ft. in diameter.
While prior art collectors and the collector of this invention both have paraboloidal reflective surfaces and achieve the same results, the use of inexpensive insulating materials, new fabricating methods, and different means to define the reflective shape may result in substantial reduction of manufacturing time as well as cost.
For example, by using special cutting and marking (or scoring) means described hereinafter, parabolic sections can be cut to very close tolerances at speeds of about 250 rpm. Subsequent shaping and cutting may be required, but can be done on pieces that are separated by transverse cuts. The parabolic support sections for a 33 ft. diameter paraboloid can be manufactured on a 96" wide corrugating machine in about 28 minutes. These support sections may be about 4 ft. high by about 16 ft. maximum length, and are well within accepted shipping dimensions that avoid special routing.
Since all components of the reflector are pre-fabricated in a factory and shipped for field assembly, it will become evident hereinafter that the highest degree of fabricated accuracy can be maintained after completion of field assembly.
Apart from the benefits of high-speed fabrication of accurately formed components, certain functional benefits accrue from the use of insulating members to form the paraboloid shape. Within the range of ambient temperature extremes and the small additional heat gain by absorption of non-reflected solar rays, insulating materials have excellent dimensional stability and therefore, the accurately-assembled paraboloidal reflector is not prone to distortions due to temperature differences that occur when clouds pass over or when portions of the disc reflector become shaded.
Therefore, it is an object of this invention to provide a paraboloidal disc solar ray reflector wherein the completed reflector is a combination of specially fabricated parts which can be assembled in the field.
It is a further object of this invention to define an accurately-focused solar ray reflector that can be field-assembled from accurately-made parts.
Further, it is an object of this invention to provide a reflector of insulating materials in order to avoid distortions and inaccuracy due to temperature differentials when subjected to full sunlight as opposed to partial shade or no sunlight.
It is a further object of this invention to provide a disc reflector made from inexpensive and abundant materials.
It is a further object of this invention to provide a disc reflector that can be fabricated and erected in a few days as opposed to many months of special fabrication.
A further object of this invention is to provide a disc reflector made of lightweight materials.
It is a further object of this invention to provide a coacting assembly of inverted-mating, parabolic shapes which can be readily installed to protect the paraboloidal shape in the event of hail storms.
It is a further object of this invention to provide paraboloidal disc reflectors having a total diameter substantially larger than currently known art devices by utilizing more than 1 pair of parabolic segments.
Above all, it is the primary object of this invention to provide a disc reflector at low cost so that numerous small power units can be remotely located for power decentralization.
With the above and other objects in view, more information and understanding of the present invention may be achieved by reference to the following detailed description.